Turbine engines, and particularly gas or combustion turbine engines, are rotary engines that extract energy from a flow of combusted gases passing through the engine onto a multitude of turbine blades. Air can be pressurized in a compressor during operation. The air channeled through the compressor can be mixed with fuel in a combustor and ignited, generating hot combustion gases which flow through turbine stages that extract energy therefrom for powering the fan and compressor rotors and generate engine thrust to propel an aircraft in flight or to power a load, such as an electrical generator.
The compressor includes a rotor assembly and a stator assembly. The stator assembly includes a plurality of circumferentially spaced apart stator vanes or airfoils that direct the compressed gas entering the compressor to the rotor blades. The stator vanes extend radially between an inner band and an outer band. A gas flow path through the stator assembly is bound radially inward by the inner bands, and radially outward by outer bands. The stator vane segments are mounted within a compressor casing. A vane stage includes a plurality of circumferentially arranged vane segments, with each segment having a plurality of airfoils extending between an arcuate inner band and an arcuate outer band.
In some designs, the vane segments are supported solely at its outer band since a conventional annular seal member is disposed between rotor stages, preventing stationary support of the inner band as well. Accordingly, the airfoils in these vane segments are cantilevered from the outer band support which creates bending moments due to the fluid flowing between the vanes which must be suitably reacted or accommodated through the outer band. The bending moments in some of these airfoils may be significant since they are supported solely at their outer band, with the inner band thereof being unsupported.
During engine operation, the gas flow through the flow path induces mechanical, thermal, and aerodynamic loads on the airfoils. Some of these loads are transmitted by the airfoils to support structures in the engine through the outer bands that are coupled to the airfoil and reacted by the support structures. In some designs, the inner bands may also, similarly, transmit some of the loads applied on the airfoils by the gas flow and reacted by other support structures. Within at least some conventional gas turbine engines, the stresses in the airfoil near the interface with the outer band and near the support structures may become large enough to cause distress in the airfoil. Under sufficiently large stresses, cracking may occur in the airfoils within the airfoil assembly near support structure locations that react the loading applied to the airfoils in the airfoil assembly by the gas flow. Designing with additional thickness at these high locations may not be possible for several reasons, such as, aerodynamic considerations, flow modifications, additional weight, and changed dynamic characteristics of the vanes and/or excessive leakages in the airfoil assemblies.